Field of the Invention
The present invention relates to a procedure and an apparatus for the control of the speed and acceleration of a hoisting motor, driven by a frequency converter (when an a.c. motor used as a hoisting motor) or by a rectifier (when a d.c. motor is used), said frequency converter or rectifier being connected to mains power source and controlled by a control unit.
Many problems are encountered in driving the hoisting motor of an elevator when an undervoltage condition appears in the mains power source. Since the torque of the motor is proportional to the square of the supply voltage, the motor cannot produce a full torque in undervoltage conditions at full speed. In this situation, the motor is unable to accelerate the elevator according to the speed reference, leading to the saturation of the controllers and, in the worst case, to an interruption in the operation of the elevator. If the motor has to produce a full torque in undervoltage conditions, the current will increase correspondingly. This may lead to overcurrent tripping.
No solution to this problem has generally been provided, but interruptions in elevator operation are common in cases where the power supply is too low, or subject to frequent and large voltage variations. A possible solution is to use an overrated motor having high enough parameters to ensure that the motor is able to produce a sufficient torque even in undervoltage conditions.
A drawback with an overrated motor is its high price, which is why this solution is not generally used. Therefore, a voltage reduction of only 5% is considered in the motor selection.